Prior to the present invention, phenolic imine compounds were prepared by the so-called Gibbs Reaction. H. D. Gibbs, Chem. Review 13, 291-319 (1927). See also D. Svobodova, et al., Mikrochimica Acta, pp. 251-264 (1978). These references, the contents of which are incorporated herein by reference, describe the coupling of phenols with imines in accordance with ##STR2##
While such compounds are useful as reporter substances, the presently disclosed compounds are resistant to interference in ion determination caused by the presence of protein, such as albumin, in the test sample.